Drill bits adapted to bore through rock for the installation of roof mounts in mines and the like have a carbide blade mounted in a transverse slot at the distal end of a drill body such as an elongate hollow tubular body. The drill body has access ports positioned near the blade, and the ports communicate with the inner bore of the body. Such blades are adapted to drill holes having a diameter of one inch and larger into the stone roof of a mine. A vacuum is drawn through the hollow drill body to draw stone cuttings removed by the blade through the ports, through the hollow shaft of the drill bit, and to a collection location remote from the drilling. Alternately, a water flushing system may be used in which pressurized water is forced through the drill body, and out ,through the ports to flush the cuttings out the mouth of the hole.
To withstand the wear incurred as such drill bits bore through stone, the blades of such drill bits are typically made of tungsten carbide and have a hardness in the range of 89.0 to 91.0 on the Ra hardness scale. A relatively small increase in the hardness of the tungsten carbide of the blade will substantially extend the useful life of a drill bit, i.e., enable the drill bit to bore through substantially more stone before it becomes unusably dull. For example, a first drill blade which is harder than a second blade by a hardness rating of 0.5 on the Ra hardness scale may be capable of drilling through 50 percent or more stone than the second drill blade before becoming unusably dull.
On the other hand, harder carbide blades are more brittle than softer tungsten carbide blades and tend to fracture as a result of the stresses upon the blade, and as a result, the blades of drills used in mines usually have hardnesses closer to 90.0 than to 91.0. It has been found that tungsten carbide blades typically fracture along the central longitudinal axis of the blade as a result of the thrust forces applied during the drilling.
A drill blade attached to a drill bit and used to bore a cylindrical hole is subjected to a number of forces. At the center of the drill blade, the thrust forces arising from the drill being forced into the hole are maximized. At the outer edges of the blade, on the other hand, strong shear forces are developed as the blade removes stone to extend the hole. The cutting edges of drill blades generally wear most noticeably at the outer ends rather than at their centers because the blade speed and the sheer forces are greater at the ends of the blade than at the center.
It would be desirable to have a blade which is very hard at the outer edges thereof so as to resist the wear as the blade bores into stone, and to have a softer, less brittle portion in the center thereof where it is subjected to the maximum thrust forces.
Other factors also affect the useful life of a drill bit. If, for example, the blade is not centered on the body of the drill bit, it will be subjected to uneven forces causing stress and breakage of the blade. It would also be desirable to provide an improved drill bit and drill blade therefor which would be more easily centered upon the body of a drill bit and which would thus reduce the incidence of fracture along the centerline thereof.